ARTICLE IN PRESS

Energy Policy 37 (2009) 2469–2474

Contents lists available at ScienceDirect

Energy Policy

0301-42

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/enpol

Viewpoint

What energy levels can the Earth sustain?

Patrick Moriarty a,�, Damon Honnery b

a Department of Industrial Design, Monash University, P.O. Box 197, Caulfield East 3145, Vic., Australiab Department of Mechanical and Aerospace Engineering, Monash University, P.O. Box 31, 3800 Vic., Australia

a r t i c l e i n f o

Article history:

Received 12 February 2009

Accepted 3 March 2009Available online 2 April 2009

Keywords:

Forecasting

Fossil fuels

Climate change

15/$ - see front matter & 2009 Elsevier Ltd. A

016/j.enpol.2009.03.006

esponding author. Tel.: +613 9903 2584; fax:

ail address: patrick.moriarty@artdes.monash.e

a b s t r a c t

Several official reports on future global primary energy production and use develop scenarios which

suggest that the high energy growth rates of the 20th century will continue unabated until 2050 and

even beyond. In this paper we examine whether any combination of fossil, nuclear, and renewable

energy sources can deliver such levels of primary energy—around 1000 EJ in 2050. We find that too

much emphasis has been placed on whether or not reserves in the case of fossil and nuclear energy, or

technical potential in the case of renewable energy, can support the levels of energy use forecast. In

contrast, our analysis stresses the crucial importance of the interaction of technical potentials for annual

production with environmental factors, social, political, and economic concerns and limited time frames

for implementation, in heavily constraining the real energy options for the future. Together, these

constraints suggest that future energy consumption will be significantly lower than the present level.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Modern industrial economies depend critically on energy,although conventional economists have been slow to recognise itsimportance. Continued economic growth would seem to requirefurther increases in energy use, or ‘useful work’ (Ayres, 2008a, b).Increases are also needed to overcome declining soil fertility(fertiliser manufacture), declining fresh water availability (irriga-tion, desalination) and declining availability of high-qualityreserves of both mineral ores and fossil energy. Much higherglobal energy use would also be necessary if industrialisingeconomies are even to approach the per capita levels of OECDcountries (Moriarty and Honnery, 2008). For these reasons,most official projections of global primary energy use over the21st century assume that it will be greater – usually much greater– than the present value of roughly 500 EJ (EJ ¼ exajoule ¼ 1018 J)(International Energy Agency (IEA), 2008a). Following IEA con-ventions, in this paper energy generated from renewable primaryelectricity sources such as hydroelectricity or wind is converted toprimary energy on a one-to-one basis, and primary energyincludes non-commercial fuel wood.

Table 1 gives the range of values for global primary energy usein the various scenarios developed in recent reports by the IEA(2008a), the US Energy Information Administration (EIA, 2008),the World Energy Council (WEC) (Schiffer, 2008), the EuropeanCommission (EC, 2006) and the International Atomic Energy

ll rights reserved.

+613 99031440.

du.au (P. Moriarty).

Agency (IAEA, 2008). What is remarkable is the small variation inenergy use for a given future year, both between the variousscenarios in any one study, and also between the different studies;even in 2050 the maximum range is only 26%. Presently, fossilfuels have an 81% share of primary energy, renewable energy (RE)13% and nuclear energy 6% (IEA, 2008a). The various studies agreethat out to 2050 at least, fossil carbon energy sources will remaindominant, with roughly an 80% share of total primary energy.Clearly, these official sources believe that future availability offossil fuels will not be a problem.

Primary rather than secondary energy is the main focus of thispaper, not only because most official projections are given in suchenergy terms, but also because the environmental impacts ofenergy use vary more closely with primary energy. It is also therelevant value to consider when discussing fossil fuel depletion.Total primary energy supply is made up of global production7stock changes. Secondary energy (or total final consumption asIEA terms it) is the sum of consumption by the different end-usesectors (excluding backflows from the petrochemical industry). In1973 the ratio of secondary energy to primary energy was 0.76,but this ratio had fallen to 0.69 by 2006 (IEA, 2008a), mainlybecause of the rising share of electricity in global final energydemand. The EC study (EC, 2006) projects that the ratio willfurther fall to 0.66 in 2030, and 0.61 in 2050, as electricity furtherincreases its share. In this paper we likewise assume that theprojected figures in Table 1 imply total final consumption of 66%in 2030 and 61% in 2050 of these primary energy values.

The main aim of this paper is to determine whether or not highlevels of primary energy are likely to be available to us in thedecades to come. We analyse future prospects for fossil fuels, RE

ARTICLE IN PRESS

Table 1Global primary energy consumption projections, 2020–2100 (EJ).

Organisation/Source 2020 2030 2050

EIA (2008) 609–677 666–807 NA

IAEA (2008) 588–655 679–826 NA

IEA (2008a) NA 661–742 NA

EC (2006) 571–608 649–706 821–933

WEC (Schiffer 2008) 616–674 701–847 846–1151

P. Moriarty, D. Honnery / Energy Policy 37 (2009) 2469–24742470

and nuclear energy in turn, and find that too much emphasis hasbeen placed on the availability of adequate reserves (or technicalpotential in the case of renewable energy) in assessing the likelylevels of energy use in the future. In contrast, our analysis stressesthe crucial importance of the interaction of technical potentialsfor annual production with environmental factors, social, political,and economic concerns, and limited time frames for implementa-tion, in heavily constraining the real energy options for the future.Together, these constraints suggest that energy consumption willbe significantly lower than the present level.

2. Hydrocarbon fossil fuels

World consumption of fossil fuels in 2007 totalled 409.0 EJ,comprising 165.5 EJ for oil, 133.0 EJ for coal, and 110.5 EJ fornatural gas (BP, 2008). Future fossil fuel use faces two uncertain-ties: first, the extent of recoverable reserves, both proven and yet-to-find, and, second, how much of these finite reserves to use eachyear. Discussions about proven recoverable oil reserves andannual production rates need a consistent terminology. Here wefollow the definitions of the Association for the Study of Peak Oil(ASPO) and include as non-conventional oil not only heavy oils(e.g. oil sands and shale oils) but also polar and deep-water oil,and natural gas liquids. This wider definition seems reasonable,since deep-water oil is reported to need a price of $US 70/barreland oil sands $US 90/barrel to ensure a reasonable rate of returnon investment (ASPO, 2009). By this definition, global productionof conventional oil may already have peaked several years ago.Indeed, ASPO consider that production of all oil peaked in 2007,and that production of both oil and gas combined (bothconventional and non-conventional) will peak around 2010 (ASPO,2009).

Most official organisations would agree with ASPO that the eraof cheap oil is over—or will only occur under global economicrecession conditions, when demand is weak. But even if it isacknowledged that conventional oil might be nearing (or evenhaving reached) peak production, official organisations are farmore optimistic on non-conventional oil. Thus the IEA (2008b)state: ‘The total long-term potentially recoverable oil-resourcebase, including extra-heavy oil, oil sands and oil shales (anotherlargely undeveloped, though costly resource), is estimated ataround 6.5 trillion barrels.’ [about 40,000 EJ]. Similarly, natural gashas very large non-conventional sources (coal seam methane,‘tight’ gas, and even methane hydrates). For coal, estimatedreserves of lower calorific value ‘sub-bituminous coal and lignite’exceed those for ‘anthracite and bituminous coal’. Althoughcombined reserves are only 18,000 EJ (BP, 2008), the ultimatelyrecoverable reserves are usually thought to be many times higher(e.g. Sims et al., 2007). Other researchers (Energy Watch Group,2007; Nel and Cooper, 2009) argue that ultimately recoverablecoal reserves have been greatly exaggerated. If these lower reserveestimates, and those for oil and gas by ASPO are true, thencombined fossil fuel production will peak in a couple of decades

(Moriarty and Honnery, 2009). In any case, the monetary, energyand environmental costs of unconventional fossil fuel extractionare all likely to be high.

Adequate global reserves of fossil fuels are a necessary but byno means sufficient condition for ensuring rising production inthe coming decades. The IEA (2008b) stress that lack ofinvestment in new production capacity, particularly given thecollapse of oil prices toward the end of 2008, could lead to lower-than-expected future oil output. Globally, before the recentdemand slump, spare capacity was at a very low level, and largeincreases in new capacity are needed, not only to allow for addedgrowth in world output foreseen by the IEA, but more importantly,to replace capacity losses as mature oil fields decline. The IEA nowrecognise that existing field output is declining much faster thanexpected (IEA, 2008b). Much of the production development willneed to be for non-conventional oil supplies, with their heavydemands for capital. Capacity increases will also be needed inother parts of the oil supply chain, including tankers, pipelines,and refining capacity.

Export reductions will occur if the domestic consumption ofOPEC countries continues its rapid rise of recent years (BP, 2008).Reductions could also occur through export restrictions by fuelexporters, as such countries can and do restrict exports forpolitical and economic reasons. OPEC has recently cut output in anattempt to support oil prices, but in the past has also restricted oilexports for political reasons. Similarly, Russia has occasionallyrestricted natural gas exports to other European countries. Fear ofthese restrictions has prompted interest in ‘energy security’ inimporting countries. Oil is the only resource for a number of oilexporting nations. Restricting production now can both maintainoil prices while also leaving more for future production andexports. A further reason for export constraint is that petroleum infuture may be far more valuable as a feedstock for plastics than asa fuel. Future restrictions could even occur with gas and coalexports. Production constraints could also be in the long-terminterests of wealthy importing countries, given that it wouldenable a steady stream of imports for centuries.

Finally, future global production of fossil fuels could fall wellbelow present levels simply because of a drop in demand, as in thepresent (2009) economic downturn. Several researchers (Hall andKlitgaard, 2006; Ayres, 2008a,b) have examined the strongcorrelation between GDP and primary energy consumption, thecorrelation becoming even tighter when corrections for changingenergy quality are made. If this strong link continues to hold, largereductions in primary energy use will lead to correspondingreductions in GDP.

The various environmental impacts of fossil fuel productionand combustion form another set of factors that could limit theannual supply of fossil fuels. Greenhouse gas atmosphericemissions from the heavy use of fossil fuels projected in Table 1would need to be drastically reduced by sequestering the CO2. Yetcarbon capture from existing power plants is expensive andenergy intensive, thus reducing the delivered energy for a giveninput of primary energy (Moriarty and Honnery, 2009). For theprojected values of secondary energy assumed above, use ofcarbon capture (especially air capture) and storage will onlyhasten depletion of fossil fuel reserves, because primary energyneeds would then be much higher than given in Table 1. Evenwithout carbon sequestration, maintaining the assumed second-ary to primary energy ratios will be impossible if the shares ofnon-conventional oil and gas, and lower calorific value coal rise, orif coal-to-oil plants are built. Also, much of the coal in placeconsists of deeply buried seams, or thin seams. In both cases, theoverburden per tonne of coal produced will be high (and with itthe monetary and environmental costs), if strip mining, thecheapest option, is used.

ARTICLE IN PRESS

P. Moriarty, D. Honnery / Energy Policy 37 (2009) 2469–2474 2471

In summary, fossil fuel production and use in future could beconstrained by geological depletion of economically recoverablefuels, economic or politically imposed limits to annual productionfrom energy-exporting nations, and environmentally imposedlimits on fossil fuel use. There is, of course, a potential solution topossible future supply shortfalls of fossil fuels: many argue (seee.g. Momirlan and Veziroglu, 2005; de Vries et al., 2007; Simset al., 2007; Matsui et al., 2008; Forsberg, 2009) that renewable ornuclear energy (or both together) can more than compensate forany shortfalls. By this logic, it would then be in the interests offossil fuel exporters to produce as fast as possible, to avoid beingleft with an unsaleable product. In the next two sections we arguethat alternatives to fossil fuels are insufficient to allow us tocontinue on the energy path forecast in Table 1.

3. Renewable energy

In 2006, total production of modern forms of RE was around17.0 EJ, with the largest being hydropower (11.1 EJ) and modernbiomass (4.6 EJ). Most RE was from fuel wood in developingcountries, estimated to be about 45 EJ. The average annual growthrate for modern RE was around 2.5% from 1980 to 2000, but sincethen has risen to about 3% (BP, 2008). If we assume that this3% growth rate continues out to 2030, modern RE would thentotal 33 EJ, roughly twice its 2006 value. Higher growth rates areunlikely because of expected slower growth in hydro and modernbiomass, which account for nearly 95% of the total. In a 2007 IEAassessment, the scenario most favourable for RE also assumed anapproximate doubling in modern RE by 2030. In contrast, globaluse of fuel wood was expected to decrease in the coming decades,displaced by modern energy sources (IEA, 2007).

Estimating future RE potential is more than usually compli-cated for several reasons. First, it often depends on technologywhich is not yet commercially proven. Although hydropower,biomass combustion and conventional geothermal power aremature technologies, their technical potential is limited (Pimentelet al., 2002; Moriarty and Honnery, 2007a,b, 2009). On the otherhand, the RE source with by far the largest potential, direct solar,awaits fundamental technology breakthroughs for unit costs to bereduced to anywhere near existing levels. A very rough calculationillustrates the scale of the cost problem. A US report estimatedthat the global solar electricity market in 2005 was in excess of$US 10 billion/year (Lewis, 2007). In 2005, solar electrical energyoutput was 0.01 EJ. Supplying all the roughly 500 EJ of primaryused globally each year with solar energy would thus cost $US 500trillion, an order of magnitude larger than the 2006 world grossnational income (World Bank, 2008). These costs in turn suggestthat total input energy costs are much higher than usuallycalculated, resulting in little or no net energy. Already, the currenteconomic downturn is adversely affecting PV cell sales (Sander-son, 2009).

Like solar, large scale use of wind also presents challenges,although wind turbine technology is by now well developed.Current global wind energy production is less than one EJ, farbelow our estimated global technical potential of 229 EJ. Realizingthis potential would require construction of over 24 million 2 MWturbines, roughly 500 times as many 2 MW units as are currentlyneeded. Further, large scale use of wind and solar will requireenergy storage and conversion, again with further technicalchallenges—and added costs. Converting electricity to hydrogenwhich could be stored, typically results in a 45% loss (Honnery andMoriarty, 2009).

Second, possible limits to RE technical potential could arisefrom the finite nature of some of the material inputs necessary fora given RE. This is particularly the case for solar PV cells, where

several options are foreclosed because of the limited globalreserves of needed metals (Feltrin and Freundlich, 2008). Third,ongoing climate and other environmental changes – includingland-use changes – will likely adversely affect future RE technicalpotential and especially individual project viability, as detailed forhydro and biomass in our previous papers (Moriarty and Honnery,2007a,b, 2009).

The various forms of RE are very sensitive to adverseenvironmental impacts, since their assumed environmentaladvantages are an important reason for preferring them overfossil fuels. Not only could ongoing environmental and land-usechanges reduce overall RE technical potential, but all RE sourcesthemselves can have potentially serious environmental impactswhen deployed on a large scale (see, e.g. Abbasi and Abbasi, 2000;Pimentel et al., 2002; Trainer, 2007; Babir, 2008; Cowern and Ahn,2008; Makarieva et al., 2008; Schroder, 2008). The natural worldfreely provides humans – and all other living organisms – with avariety of ecosystem services, which are vital for our and theircontinued existence. For humans, the obvious ones are theprovisioning services of food, fibre, lumber and fresh water. Thesein turn depend on a variety of other less-obvious regulating

ecosystem services, including pest control, plant pollination andair and water quality (Millennium Ecosystems Assessment, 2005;Carpenter et al., 2009). Widely deployed, RE can act to undermineecosystem services.

Equally important, the various provisioning ecosystem servicescan be in conflict with each other. Particularly in the case ofbiomass, the expansion of renewable energy can compromise theprovision of food, forestry and other fibre products, as an analysisof terrestrial net primary production (NPP) shows. NPP is ameasure of the net conversion of atmospheric CO2 by photo-synthesis into plant biomass (mainly natural vegetation) over agiven time period, and is obtained by subtracting the autotrophic(self) respiration of plants from gross biomass production.Photosynthesis on land produces an annual total NPP of roughly120 billion tonnes dry matter, or 1900 EJ (Moriarty and Honnery,2007a; Field et al., 2008).

Kleidon (2006) has argued that attempts to increase the globalhuman appropriation of NPP (HANPP) much above his present-day estimate of 40% are self-defeating. His simulated results, using‘a coupled dynamic vegetation–climate system model of inter-mediate complexity’ show that as the HANPP fraction grows, theabsolute value of HANPP (in terms of gC/m2/day, for example) willbegin to fall after reaching about 45%. In other words, since outputof food, forestry and other fibre products will need to be expanded(because of ever-rising human numbers) in a world whereongoing climate changes could adversely affect output, thepotential for sustainable bioenergy production could be close tozero. Likewise, Field et al. (2008) estimate that only 27 EJ ofbiomass energy can be harvested globally without underminingfood production or worsening climate change. Further, cellulosicbiomass may never be a feedstock for liquid fuels; a recent paper(Felix and Tilley, 2008) found that conversion of switchgrass toethanol may not yield net energy.

Already, global species extinction is estimated at around12,000 per year, or 0.25% per year of all Earth’s species (Aviseet al., 2008). Any expansion of HANPP will accelerate this speciesloss. Although biomass and hydro have the most adverse effectson the environment and its continued ability to deliver ecologicalservices, all RE sources can be expected to have some negativeeffects. The negative effects of biomass and hydro may in partsimply result from their already-widespread deployment, andthe required 3–4 orders of magnitude scale-up of wind andsolar energy could well reveal similar problems. As an example,although much has been made of bird kills by wind turbines, batkills are far more frequent. It appears that migratory tree-roosting

ARTICLE IN PRESS

P. Moriarty, D. Honnery / Energy Policy 37 (2009) 2469–24742472

species are most at risk, but why they are killed in such numbersis uncertain (Kunz et al., 2007). These insect-eating bats helpprovide an important ecosystem service—pest control. Just asimportant as environmental effects, RE can also endanger humanlife. Kerr and Stone (2009) report recent research which arguesthat the filling of the Zipingpu hydro dam may have beenresponsible for the devastating 2008 Sichuan earthquake. Thefilling of the Koyna Reservoir in India, was judged responsible for a1967 earthquake which killed 200 people.

Although citizens’ opposition to various forms of RE installa-tions often focuses on environmental issues to press their case, itcan also result from more personal concerns. Less-pressing,perhaps, are worries that local construction of wind farms mightlower real estate prices or spoil the view. But, particularly withhydro schemes in the developing countries, opposition is oftenmotivated by well-founded fears of losing one’s home, livelihoodand way of life. Such opposition arises partly because of the largeland requirements for most forms of RE compared with the othertwo energy sources (Pimentel et al., 2002).

Future use of RE is thus highly uncertain, for a variety ofreasons. The optimistic technical potentials (often many thousandEJ) reported in the literature (e.g. Momirlan and Veziroglu, 2005;de Vries et al., 2007) not only depend on unprecedentedtechnological advances, but also assume that no environmentalchanges adverse to RE production occur. Further, RE sources, likeother energy sources, can in turn have serious environmentalconsequences, potentially undermining both their political sup-port and the environmental services the natural world provides.

4. Nuclear energy

At the end of 2007, some 439 nuclear power plants were inoperation around the world, with a total generating capacity of372 gigawatt (GW) supplying 9.4 EJ of electric power in 2007(IAEA, 2008). Global proven reserves of uranium are 2.85 milliontonne (MT); ultimately recoverable conventional resources areestimated at 17.1 MT. If, in the extreme case, present-day thermalreactor types had to provide 1000 EJ annually, even 17.1 MT wouldonly last for about 10 years and 2.85 MT for less than two years(Moriarty and Honnery, 2007a). On the other hand, at presentrates of annual nuclear power output, 17.1 MT would last theworld for several hundred years.

The IAEA (2008), in assessing the future of nuclear energy,projected that in the low-growth case, nuclear energy’s share ofglobal electricity generation in 2030 would fall from its 2007value of 14.2% to 12.4%. Even in their high-growth scenario,nuclear’s share would only increase to 14.4%, just above itspresent level. The EIA (2008) high- and low-growth projectionsare very similar to those of the IAEA. Both sets of forecasts weremade before the current global economic downturn, and morerecently one nuclear consultant predicted that ‘the industry willnot even be able to replace the units being shut down becauseof ageing’ (Brumfiel, 2008a). The prospects for rapid growthin nuclear power are not helped by escalating construction costs;in the US the cost of planned 1 GW nuclear plants is as high as$US 10 billion (Romm, 2008).

Given this low anticipated output of nuclear energy, fuelconstraints are unlikely to restrict output. However, the lowgrowth of nuclear power in recent decades, particularly inWestern Europe and North America, is not only a consequenceof high costs. Political opposition has led a number of countries toveto nuclear power plants, and others, such as Germany,to commit to phasing out existing programs. Other parts of thefuel cycle may also face opposition; the state of Nevada in the

US opposes construction of a nuclear waste repository at YuccaMountain.

Although opposition to nuclear power is usually labelledenvironmentalist, nuclear operations, with the important excep-tion of uranium mining, have few direct effects on earth’s energyand material flows, or ecosystems. Negligible greenhouse gasemissions are released during reactor operation, although they areincurred in other parts of the fuel cycle, especially in reactorconstruction and uranium enrichment. Instead, opposition hasbeen based largely on the health and safety risks to humans.These can arise from human error, as in the Chernobyl reactoraccident, from deliberate human actions, such as the diversion offissile materials for nuclear weapons, or from natural hazards.Although fires, floods and even tsunamis could conceivably affectreactor safety, earthquakes are the main hazard. In 2007, theworld’s largest nuclear power complex at Kashiwazaki-Kariwa inJapan, was struck by a 6.6 magnitude earthquake. The sevenreactors were undamaged, but the fault line was previouslyunknown to the plants’ designers. The reactors have not yet beenrestarted (Sacchetti, 2008).

Up to 2030, only thermal reactors, chiefly modified versions ofexisting light water reactors, are expected to be in operation.Beyond 2030, breeder reactors are a possibility. Breeder reactorscould in principle extend uranium reserves by a factor of 30, butface severe technical problems given their high operatingtemperatures, and significantly increased risk of nuclear prolif-eration (Moriarty and Honnery, 2007a, 2009). The breeding rate ofnew fissile material (and costs and difficulties of fuel reproces-sing) could be the limiting factor on the rate of expansion. Further,like all fission reactor types, they contain a large inventory ofhighly radioactive materials, which could conceivably be releasedby a reactor accident or sabotage. Their risks to human health andsafety would overall be much greater than for thermal reactors.

Research on fusion energy has been in progress for half acentury, but fusion is still nowhere near commercial realisation,which, if it ever occurs, will not be before the last quarter of thiscentury. The favoured approach is confining deuterium–tritiumplasma in a toroidal magnetic field. While this approach may besuperior from a physics viewpoint, the engineering problems aredaunting, mainly because of the intense neutron flux and its effecton the blanket-shield and the reactor structural materials(Parkins, 2006). Electricity costs will be higher, possibly manytimes higher, than for existing fission-based electricity (Hirsch,2003). With its rising capital costs and lack of private investorinterest, nuclear power of any type will be struggling to maintainits existing output levels.

5. Policy implications

Most energy analysts think that global primary energy use inthe decades to come will be much higher than present levels, withprojections for the year 2050 often 1000 EJ or more. Even the IEA(2008b) projections, which take into account the current globaleconomic downturn, foresee 2030 primary energy of 712 EJ in thebase case. Yet the energy sources that singly or together mustmeet this projected demand – fossil fuels, RE, and nuclear power –all face three categories of possible limitations, namely:

�

Physical limits such as geological limits on fossil fuel reserves. � Political, economic, technical or social constraints on their

production.

� Environmental constraints on their production or use.

These categories are not independent of each other, but caninteract. Fears of impending fossil fuel reserve depletion could

ARTICLE IN PRESS

P. Moriarty, D. Honnery / Energy Policy 37 (2009) 2469–2474 2473

encourage either energy-exporting or -importing countries tolimit production/use below that possible at that time. Similarly,environmental problems from energy production or use (espe-cially CO2 emissions from fossil fuels) could hasten moves bycitizen groups or policy-makers to limit its use.

Advocates for high future energy use often acknowledge thatmuch of this energy will need to come from technologies not yetcommercial, and sometimes not even at the demonstration stage.They can point to the remarkable technical advances over the pastcentury as reasons for optimism, but such optimism about newenergy sources may be misplaced, for several reasons. First, incontrast to advances in information technology, experience showsthat new energy sources are usually more costly than anticipated,and take much longer to gain a significant market share. Second,new technologies will increasingly be subjected to constraints ontheir use: for instance, they may require materials in scarcesupply, their large-scale deployment may uncover novel environ-mental problems, or ongoing changes in global land-use andclimate may reduce their potential for use.

Our analysis so far suggests that even holding primary, andeven more so delivered energy, at present levels in 2050 will provedifficult. Several implications for policy follow. First, we will needto avoid as far as possible the use of non-conventional fossil fuels,since these have higher than average primary/secondary energyratios, and thus higher environmental damages, includingCO2 emissions per unit of secondary energy. A second, related,point is the need to avoid carbon capture (especially air capture)and sequestration, as this will likewise increase the primary/secondary energy ratio. Attempts to maintain, let alone raise,secondary energy output will thus also hasten fossil fueldepletion. We have argued elsewhere that restricting annual useof primary energy from fossil fuels to 50–100 EJ would bothremove the need for carbon capture and storage and also permituse of fossil fuels for several centuries (Moriarty and Honnery,2009). It would also reduce the need for large increases in non-conventional fossil fuel use.

The third policy implication is that we should delay for as longas possible the need for conversion and storage of intermittentRE sources—mainly wind and solar energy. We have so far onlydiscussed energy from a global viewpoint, but, of course,countries differ greatly in their energy resource endowments. Ifmodern RE only doubles by 2030, as suggested above, andelectricity use grows as forecast, grid integration of intermittentRE is unlikely to be a serious technical problem in any country. Butgiven the limited potential of baseload RE, the cost and riskproblems facing nuclear fission expansion, and the desirability –and perhaps need – to reduce fossil fuel use, the share ofintermittent electricity in grids could grow rapidly. Also, if wind/solar electricity expand more rapidly than anticipated, the needfor conversion/storage will be brought forward; if this is notpossible, electricity grid management practices will need to bechanged. Without either of these two interventions, the nature ofthe existing electricity grid will ultimately limit intermittent REproduction.

The current use of fossil fuels brings about another problem.Since fossil fuels dominate our existing energy system, they willhave to power any possible shift to RE or nuclear sources. Theeffect this has on fossil fuel reserves will depend largely on thesize and speed of the shift, since fossil fuel energy used will beadditional to that already in use. If changes to climate warrant adecisive shift to RE within the next decade, this shift would notonly hasten depletion of fossil fuels, but could also hasten climatechange. If the shift to RE was powered by RE alone, it is likely therate of change would be too small; we would possibly seesignificant climate problems or face fossil fuel depletion before REcould supply large amounts of primary energy.

Clearly, there are no easy choices facing future production anduse of energy for most countries. Even if fossil fuel reserves arecloser to the optimist position, there is no guarantee that politicalor environmental constraints, particularly the need for green-house gas emission reductions, will not greatly limit annualoutput. But if deep cuts are made to fossil fuel use, the resultinglarge spare capacity will slow change away from fossil fuel powerplants. Only large reductions in global primary energy use, withall its difficulties of implementation, can meet the resource,environmental, economic and political problems that futureenergy use will face.

Acknowledgement

Patrick Moriarty acknowledges the financial support of theAustralasian Centre for the Governance and Management of UrbanTransport (GAMUT) in the preparation of this paper.

References

Abbasi, S.A., Abbasi, N., 2000. The likely adverse environmental impacts ofrenewable energy sources. Applied Energy 65, 121–144.

Association for the Study of Peak Oil and Gas (ASPO), 2009. ASPO Newsletter 97(also earlier newsletters). Accessed on 6/02/2009 at /www.peakoil.netS.

Avise, J.C., Hubbell, S.P., Ayala, F.J., 2008. In the light of evolution II: biodiversity andextinction. PNAS 105 (suppl. 1), 11453–11457.

Ayres, R.U., 2008a. Sustainability economics: where do we stand? EcologicalEconomics 67 (2), 281–310.

Ayres, R.U., 2008b. Energy and economic growth. In: Babir, F., Ulgiati, S. (Eds.),Sustainable Energy Production and Consumption. Springer, Dordrecht, NL.

Babir, F., 2008. Transition to renewable energy systems with hydrogen as an energycarrier. Energy.

BP, 2008. BP Statistical Review of World Energy 2008. BP, London.Brumfiel, G., 2008a. Nuclear renaissance plans hit by financial crisis. Nature 456,

286–287.Carpenter, S.R., Mooney, H.A., Agard, J., Capistrano, D., DeFries, R.S., Diaz, S., et al.,

2009. Science for managing ecosystem services: beyond the millenniumecosystems assessment. PNAS 106, 1305–1312.

Cowern, N.E.B., Ahn, C., 2008. Thermal emissions and climate change: a nuclearproblem and a voltaic solution? Accessed on 30/12/08 at /http://arxiv.org/ftp/arxiv/papers/0811/0811.0476.pdfS.

de Vries, B.J.M., van Vuuren, D.P., Hoogwijk, M.M., 2007. Renewable energy sources:their potential for the first-half of the 21st century at a global level: anintegrated approach. Energy Policy 35, 2590–2610.

Energy Information Administration (EIA), 2008. International energy outlook 2008.US Department of Energy. Accessed on 28/01/09 at /http://www.eia.doe.gov/oiaf/ieo/pdf/0484(2008).pdfS.

Energy Watch Group, 2007. Coal: resources and future production. EWG Series no.1/2007.

European Commission (EC), 2006. World energy technology outlook—WETO H2.Directorate General for Research, EUR 22038. Accessed on 7/01/09 at /http://ec.europa.eu.research/energy/pdf/weto-h2_en.pdfS.

Felix, E., Tilley, D.R., 2008. Integrated energy, environmental and financial analysisof ethanol production from cellulosic switchgrass. Energy doi:10.1016/j.energy.2008.10.013.

Feltrin, A., Freundlich, A., 2008. Material considerations for terawatt leveldeployment of photovoltaics. Renewable Energy 33, 180–185.

Field, C.B., Campbell, J.E., Lobell, D.B., 2008. Biomass energy: the scale of thepotential resource. Trends in Ecology and Evolution 23 (2), 65–72.

Forsberg, C.W., 2009. Sustainability by combining nuclear, fossil, and renewableenergy sources. Progress in Nuclear Energy 51, 192–200.

Hall, C.A.S., Klitgaard, K.A., 2006. The need for a new, biophysical-based paradigmin economics for the second half of the age of oil. International Journal ofTransdisciplinary Research 1 (1), 4–22.

Hirsch, R.L., 2003. Fusion power: the burning issue. Public Utilities Fortnightly 141(3), 33–36.

Honnery, D., Moriarty, P., 2009. Estimating global hydrogen production from wind.International Journal of Hydrogen Energy 34, 727–736.

International Atomic Energy Agency (IAEA), 2008. Energy, electricity and nuclearpower estimates for the period up to 2030. Accessed on 12/01/2009 at /http://www-pub.iaea.org?MTCD/publications/PDF/RDS1-28_web.pdfS.

International Energy Agency, (IEA), 2007. Renewables in global energy supply: anIEA fact sheet. IEA/OECD, Paris.

International Energy Agency, 2008a. Key world energy statistics 2008. IEA/OECD,Paris.

International Energy Agency, 2008b. World energy outlook 2008. IEA/OECD, Paris.Kerr, R.A., Stone, R., 2009. A human trigger for the great quake of Sichuan? Science

323, 322.

ARTICLE IN PRESS

P. Moriarty, D. Honnery / Energy Policy 37 (2009) 2469–24742474

Kleidon, A., 2006. The climate sensitivity to human appropriation of vegetationproductivity and its thermodynamic characterization. Global and PlanetaryChange 54, 109–127.

Kunz, T.H., Arnett, E.B., Erickson, W.P., Hoar, A.R., Johnson, G.D., Larkin, R.P., et al.,2007. Ecological impacts of wind energy development on bats: questions,research needs and hypotheses. Frontiers in Ecology and the Environment 5(6), 315–324.

Lewis, N.S., 2007. Towards cost-effective solar energy use. Science 315, 798–801.Makarieva, A.M., Gorshkov, V.G., Li, B.-L., 2008. Energy budget of the biosphere and

civilization: rethinking environmental security of global renewable and non-renewable resources. Ecological Complexity 5, 281–288.

Matsui, K., Ujita, H., Tashimo, M., 2008. Role of nuclear energy in environment,economy and energy issues of the 21st century green house gas emissionconstraint effects. Progress in Nuclear Energy 50, 97–102.

Millennium Ecosystems Assessment, 2005. Ecosystems and Human Well-Being:Synthesis. Island Press, Washington, DC.

Momirlan, M., Veziroglu, T.N., 2005. The properties of hydrogen as fuel tomorrowin sustainable energy system for a cleaner planet. International Journal ofHydrogen Energy 30, 795–802.

Moriarty, P., Honnery, D., 2007a. Intermittent renewable energy: the only futuresource of hydrogen? International Journal of Hydrogen Energy 32, 1616–1624.

Moriarty, P., Honnery, D., 2007b. Global bioenergy: problems and prospects.International Journal of Global Energy Issues 27 (2), 231–249.

Moriarty, P., Honnery, D., 2008. Mitigating greenhouse: limited time, limitedoptions. Energy Policy 36, 1251–1256.

Moriarty, P., Honnery, D., 2009. Hydrogen’s role in an uncertain energy future.International Journal of Hydrogen Energy 34, 31–39.

Nel, W.P., Cooper, C.J., 2009. Implications of fossil fuel constraints on economicgrowth and global warming. Energy Policy 37, 166–180.

Parkins, W.E., 2006. Fusion power: will it ever come? Science 311, 1380.Pimentel, D., Herz, M., Glickstein, M., Zimmerman, M., Allen, R., Becker, K., et al.,

2002. Renewable energy: current and potential issues. BioScience 52,1111–1120.

Romm, J., 2008. The Self-Limiting Future of Nuclear Power. Accessed on 27/01/09at /www.americanprogressaction.orgS.

Sacchetti, D., 2008. Earth, wind and fire: preparing nuclear power plants fornature’s fury, IAEA Bulletin, 50–1, 50–53.

Sanderson, K., 2009. Not so sunny after all. Nature 457, 362–363.Schiffer, H.-W., 2008. WEC energy policy scenarios to 2050. Energy Policy.Schroder, W.U., 2008. Energy realpolitik: towards a sustainable energy strategy.

Accessed on 5/12/2008 at /http://arxiv.org/ftp/arxiv/papers/0804/0804.2159.pdfS.

Sims, R.E.H., Schock, R.N., Adegbululgbe, A., Fenhann, J., Konstantinaviciute, I.,Moomaw, W., et al., 2007. Energy supply. In: Metz, B., Davidson, O.R., Bosch,P.R., Dave, R., Meyer, L.A. (Eds.), Climate Change 2007: Mitigation. CUP,Cambridge, UK, pp. 251–322.

Trainer, T., 2007. Renewable Energy Cannot Sustain a Consumer Society. Springer,Dordrecht, NL.

World Bank, 2008. Key Development Data and Statistics. Accessed on 29/01/09 at/http://web.worldbank.orgS.